The Egyptian government arrested hundreds of religious extremists as well as leftist and rightist opposition figures last night in the biggest crackdown here since the declaration of a multiparty system five years ago. It also banned six publications.

The moves came only two days before a major speech by President Anwar Sadat on Moslem-Christian violence that has left 21 persons dead and 100 wounded in the past three months. An official statement on the arrests referred to the sectarian conflict.

Sadat has promised to announce stiff measures to end the strife and opposition leaders fear he may also curb the country's troubled experiment in multiparty democracy.

It appeared from yesterday's arrests that Sadat's longtime truce with the right-wing Moslem Brotherhood has come to an abrupt end and that he is on the verge of curbing their increasingly political activities.

The official statement disclosing the arrests gave no indication of the number involved, but Mansour Hassan, minister of state for presidential affairs and information, said the figure ran into the hundreds. One report said more than 600 persons, including politicians, lawyers and journalists as well as Moslem and Coptic Christian extremists, were included.

The semiofficial Cairo daily Al Ahram said 553 persons had been arrested yesterday and another 119 were already in detention from the outbreak of sectarian violence in Cairo last June.

Among the most prominent Egyptians reportedly held were Fuad Sarag Iddin, 70, leader of the defunct liberal Neo-Wafd Party; Helmi Mourad, 62, deputy leader of the official opposition Socialist Labor Party; Mohammed Heikal, author and confidant of the late president Gamal Abdel Nasser; Hussein Abdul Razak, editor of the Socialist Labor Party's weekly Al Shaab, which was banned, and Omar Telmisani, editor of the Moslem Brotherhood monthly Al Dawaa, also banned.

The government already had banned the September issue of Al Dawaa, a slick and bitterly anti-Jewish publication of the relatively moderate brotherhood.

Today's government list of publications banned also included the brotherhood's Al Etissam, the fundamentalist Moukhtar Al Islami, and the Coptic publications Watani and Karraza.

A Socialist Labor Party spokesman said that among those arrested was Sheik Mahmoud Kiskh, a popular and outspoken right-wing religious leader in Cairo. But an aide at his downtown mosque refused to confirm this.

The blind sheik, who was exiled to Saudi Arabia for a number of years, has been delivering vitriolic attacks recently during his Friday sermons against the minority Coptic community while demanding a stricter adherence to Islamic practices. Tapes of his sermons spread his message throughout Egypt.

Despite the expectation of unrest with the announcement of the arrests, Cairo was calm today. But observers were still waiting to see what the public reaction would be at mosques during Friday's noontime prayer services.

The government said those arrested were both Moslems and Christians who had indulged in "irresponsible and suspicious acts" under the cover of religion. It said they had "joined hands with other hostile elements taking advantage of the atmosphere of freedom and democracy to achieve personal aims and interests which are detrimental to national unity and social peace and threaten the security and interests of citizens."

But Khalid Mohieddin, leader of the leftist National Progressive Unionist Party, said in an interview that the Sadat government was using the religious issue as a "cover" to strike against its political opponents of both the left and right and blame them for the sectarian strife presently afflicting the country.

"By doing this, the president is not going to the roots of the problem but trying to use this opportunity to fight national opposition elements in the country. This will not serve either political stability, democracy or the spirit of the rule of law," he said.

"Everyone who reads the names of the detainees will understand the aim of the campaign," he said.

By midday, Mohieddin and Ibrahim Shukri, head of the Socialist Labor Party, were reporting the arrest of at least 30 opposition leaders, including journalists, lawyers and politicians.

Later, reports from Alexandria said at least 40 persons had been rounded up there, including Heikal and a number of outspoken Moslem religious leaders.

In the absence of any official information on the names and number of those arrested, the leftist opposition parties were providing most of the information -- which reporters were unable to confirm immediately with other sources.

The Moslem Brotherhood was among Sadat's firmest supporters in his fight against the left early in his 11-year rule. But it and fundamentalist groups farther to the right have become increasingly outspoken in their criticism of his foreign and domestic policies, particularly his peace treaty with Israel and close alliance with the United States.

More recently, they have turned to criticizing the Coptic minority, 3 million to 6 million of Egypt's population of 42 million, and there have been a number of violent incidents. In June, Moslems and Copts clashed for three days in a Cairo district over a land dispute. At least 16 persons were killed and 50 wounded.

Last month, a bomb hurled into a Coptic wedding killed five persons and wounded 50, among them a number of Moslems.

Before leaving for talks with President Reagan, Sadat said in a speech that he was deeply concerned about the sectarian violence, which he said had reached a "dangerous level."

Sadat set out in 1976 to create a multiparty system. In 1979 elections, four parties contested in addition to a number of independent candidates. But his own National Democratic Party holds a heavy majority of the 390 seats. The largest opposition group, Shikri's Socialist Labor Party, has 17.